Hammond AO-29 Tube Organ to Guitar Amp, the "Holy Roller"

[tablebeast]

I've been storing a half dozen old tube Hammond organs for years and just got around to digging into the first couple of them this week, with the intention of making a couple of modified instrument amps. At first I looked at an M-3 from about 1955, but it had the big, silver, 380-0-380 secondary power transformer and field coil speaker. With the voltage being too high for what I'm thinking about for this first project, I'll save that one for later...

Next I looked at an M-111 from about 1966. It was more of what I wanted in a parts donor. It had a later A0-29 chassis with the smaller, black, 320-0-320 power transformer. A separate reverb amp (A0-44 chassis) with its own tank and a total of three regular AlNiCo speakers were also lurking inside. Besides repurposing the amps, reverb tank and speakers, I plan to reuse some of the wood and other bits to build the modified cabinets. I'm planning on a 2x12 combo with the A0-29 and a 1x8 combo extension reverb amp with the A0-44 (which I will document on it's own thread later).

To be clear to Hammond lovers out there, these organs were all saved from being thrown out and are rougher than they look. I love Hammonds too, and I plan to reuse as much as possible myself and then put parts I'm not using into the hands of the people that need them to keep survivors going. Anything left over after that will be made into some fun art projects, but NONE of them will be wasted! I intend to take these nearly tossed parts and make them into something interesting and innovative, while saving a couple of old units that may need hard to find parts. So no need for this to upset anyone.

If you see any parts in the pics you need (stuff that I'm not talking about using, of course) just let me know! Full plans for my "Holy Roller" amp will be revealed here soon...

 

[tablebeast]

Here is the main A0-29 chassis after taking it completely apart. What you see here are the parts culled from it for my project(s).

1. Main chassis. It will be cleaned and all the tube sockets will be used as they are. Yup, I have plans for each and every one of the 11 tube slots!
2. 2x circuit boards. I love this Hammond style. Its so easy to take apart and rework.
3. Percussion circuit transformers. I have some interesting plans for these usually unused bits of iron. The lo-to-hi-z pedal input unit will be used as passive DI. The other two are going to be used for a much more fun purpose... currently I am considering a varimu compressor! I have the room and resources in the chassis. I don't require a hi-fi frequency response for guitar anyway, so I just need to figure out a simple circuit. I'm sketching out something simple that uses a pair of 6BA6 and a 12AU7. That lets me keep and use the 7 and 9 pin sockets from the very same percussion circuit!
4. Output transformer. Output transformer meant for a pair of 6V6 in PP. This will go right back to doing what it has always done, but the output wiring won't have the weird center tapped NFB circuit.
5. Power transformer. This is the 320-0-320 version, the one with easy to tame B+ voltage.
6. Resistors. This is most of what was in there. I keep anything with long enough remaining legs and/or are over 1/2 watt. I like to have these available to use, when appropriate, usually for tone reasons.

 

[tablebeast]

Here is a link to my video series on this project.

 

[tablebeast]

Any input on what I'm doing here? Any better places online for me to share this project?

 

[shabtek]

Are there small output transformers with 5 wires on the primary?

If so, they may be the ones that have a cathode feedback tap iirc, PRR explained them well somewhere…they work really well when wired correctly, and interesting for exploration.